amd-xgbe: Replace MII_ADDR_C45 with XGBE_ADDR_C45

The xgbe driver reuses MII_ADDR_C45 for its own purpose. The values
derived with it are never passed to phylib or a linux MDIO bus driver.
In order that MII_ADDR_C45 can be removed, add an XGBE specific

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>